Generally, CRTs, transmittive liquid crystal displays with backlights are generally used in the display devices of computers and mobile devices. The displays of this type are the so-called emissive displays which include internal emission means.
Based on recent studies, it is proposed to preferably use non-emissive reflective display devices in terms of work efficiency and fatigue in reading texts, etc., on display. The reflective display device, which requires no internal emission means and uses natural light, etc. for display, is good for the eye and effective to decrease the electric power consumption.
To realize further lower electric power consumption, display devices having the memorization ability to retain displayed information even when their source power is turned off are expected.
As such display devices are proposed display devices using chiral nematic liquid crystals and cholesteric liquid crystals. Chiral nematic liquid crystals are liquid crystals comprising nematic liquid crystals and chiral catalysts added to the nematic liquid crystals. Chiral nematic liquid crystals and cholesteric liquid crystals have a characteristic of reflecting selectively light of specific wavelengths.
A proposed display device using a chiral nematic liquid crystal will be explained with reference to FIGS. 35A-B. FIGS. 35A-B are a schematic view of the proposed display device using a chiral nematic liquid crystal.
As shown in FIGS. 35A-B, a photo-absorbing layer 114 is formed on a substrate 110 of glass. An electrode 112 of ITO (Indium-Tin-Oxide) is formed on the photo-absorbing layer 114. A substrate 118 of glass is formed on the substrate 110 with the photo-absorbing layer 114 and the electrode 112 formed on, opposed to the substrate 110. An electrode 120 of ITO is formed on the side of the substrate 110, which is opposed to the electrode 112. A liquid crystal layer 122 of chiral nematic liquid crystal is provided between the substrate 110 with the photo-absorbing layer 114 and the electrode 112 formed on and the substrate 118 with the electrode 120 formed on. Thus, the display device using the chiral nematic liquid crystal is constituted.
A display device using a chiral nemtaic liquid crystal is disclosed in, e.g., Japanese published unexamined patent application No. Hei 06-507505.
Then, the operation of the display device using the chiral nematic liquid crystal will be explained.
FIG. 35A shows the planer state. In the planer state, that of the incident light, whose wavelength corresponds to a helical pitch of the liquid crystal molecules is reflected. A wavelength λ for a maximum on a reflection spectrum is expressed byλ=n·pwherein an average refractive index of the liquid crystal is represented by n, and a helical pitch of the liquid crystal is represented by p. Wavelength band width Δλ of reflected light is expressed byΔλ=Δn·pwherein an isotropy of refractive index of liquid crystal is represented by Δn.
FIG. 35B shows the focalconic state. In the focalconic state, the incident light passes through the liquid crystal layer 122 of the chiral nematic liquid crystal and is absorbed by the photo-absorbing layer 114 formed on the substrate 110. Accordingly, in the focalconic state, black color is displayed.
FIG. 36 is a graph of reflection spectra of the chiral nematic liquid crystal. The wavelengths are taken on the horizontal axis, and on the vertical axis reflectances are taken. The reflectances on the vertical axis were given when the reflection on a white reflection board is 100%.
The reflection wavelength of chiral nematic liquid crystals can be set at a prescribed value by suitably setting chiral catalyst amounts to be added to the cholesteric liquid crystals. The addition of larger amounts of chiral catalysts decreases the helical pitches p of the liquid crystals and shortens the wavelengths λ of the reflected light.
Chiral nematic liquid crystals have a characteristic of reflecting either of right circularly polarized light and left circularly polarized light. This is described in, e.g., SID 97 DIGEST, p. 1019-1022. Characteristics of the chiral catalysts to be added to the chiral nematic liquid crystals can set the chiral nematic liquid crystals to be right circularly polarized light or left circularly polarized light. Chiral nematic liquid crystals, which reflect either of the right circularly polarized light and the left circularly polarized light, theoretically has the upper limit of the reflectance of 50%.
Planer and focalconic states are retained substantially permanently unless an external force is applied to the liquid crystals. Accordingly, the use of chiral nematic liquid crystals can provide display devices having memorization ability which can retain displayed information even when their power sources are turned off.
As described above, chiral nematic liquid crystals, which can constitute reflective display devices and can retain displayed information even when the power sources are turned off, is noted as liquid crystals which will form the next generation display devices.
In a display device using a single layer of a chiral nematic liquid crystal, in the planer state, light of a wavelength corresponding to the helical pitch is selectively reflected, whereby the display colors are chromatic. On the other hand, in the focalconic state, the incident light is absorbed by the photo-absorbing layer, whereby the display color is black. Accordingly, the display device using the single layer of the chiral nematic liquid crystal can display chromatic colors or black color but cannot display white color.
Techniques of displaying white color by chiral nematic liquid crystals are proposed as follows.
Japanese published unexamined patent application No. Hei 09-503873 discloses the technique of mixing a plurality of kinds of chiral nematic liquid crystals, whereby all the visible spectra of 400-700 nm is covered to thereby display while color.
Japanese published unexamined patent application No. 2001-066627 discloses the technique of providing four liquid crystals of R (red), G (green), B (blue) and Y (yellow), whereby substantially all the visible spectra are covered to thereby display white color.
Japanese published unexamined patent application No. 2001-109012 discloses that chiral nematic liquid crystals of three colors, RGB are used to display white color.
Japanese published unexamined patent application No. Hei 11-231339 discloses that a chiral nematic liquid crystal layer which reflects selectively light of yellow color is formed on a photo-absorbing layer which absorbs light of blue color to display white color.
Techniques of display white color by using light scattering in focalconic state are also proposed.
However, the reflection wavelength band of chiral nematic liquid crystals has a full width at half maximum of about 70-110 nm. The display device disclosed in Japanese published unexamined patent application No. Hei 09-503873 cannot cover all the visible spectra only by mixing chiral nematic liquid crystals of, e.g., two kinds and accordingly cannot display white color. Furthermore, such display device, in which two or more kinds of liquid crystals are mixed in one polymer, has a risk that the liquid crystals might be mixed with one another.
The display devices disclosed in Japanese published unexamined patent application No. 2001-066627 and Japanese published unexamined patent application No. 2001-109012 both requires three or more liquid crystal layers, which is a blocking factor for the cost reduction. These display devices have high drive voltages, and their drive methods are complicated.
The display device disclosed in Japanese published unexamined patent application No. Hei 11-231339 has blue and white display colors. Accordingly, the visibility of the display devices is low unsuitable to read documents, such as texts, etc.
The technique of displaying white color by the scattering of light in focalconic state has the reflectance which is as low as about 20%, and bright white color display cannot be obtained. Accordingly, the display device using such technique cannot have high contrast.
As described above, none of the proposed techniques have been able to provide inexpensive display devices having good white and black displays.
Chiral nematic liquid crystals have a characteristic that as the observation angle is increased, the selective reflection wavelengths shift to the side of shorter wavelengths. Accordingly, in display devices using simply chiral nematic liquid crystals, hues of the display colors change depending on observation direction changes. FIG. 37 is a conceptual view of the observation angle change. For example, a display which is in red when observed at the front changes to green as the observation angle θ is increased, and changes to blue as the observation angle θ is further increased. For example, a display which is in green when observed at the front changes to blue as the observation angle θ is increased. Monitor display devices are required to have a ±60° visibility range. The hue change in the ±60° range is not preferable. Accordingly, techniques of reducing the hue changes corresponding to the observation angle changes have been expected.